Manufacturing and industrial plants typically utilize compressed air as an energy source that may be available throughout the plants. These plants use a compressed air system to provide the compressed air. The demand for compressed air is dynamic and may vary throughout the day and from day to day. Compressed air systems are typically sized (and often oversized) based on full load operating conditions and are designed to operate most efficiently at full load. These systems typically use Fixed Speed Drive (FSD) compressors, such as centrifugal compressors which may have one or more stages to achieve the rated full load output in an efficient manner. However, the demand on most compressed air systems varies throughout the day and the systems are often operating at less than full load. The loading may be adjusted based on throttling the input to the FSD compressors and any over production of compressed air may be removed by venting compressed air from the system through a blow-off valve. FSD compressors are inefficient at part loads and if not controlled properly a system could have several or all of its FSD compressors simultaneously running at part loads, crippling the overall efficiency. Additionally, venting of compressed air from the system is also inefficient.
Variable Speed Drive (VSD) compressors, such as variable speed screw compressors, use variable speed motors to modulate their output. The advantage of this is that it allows the compressor to have a relatively linear output (cubic feet per minute) to energy (kilowatt) input efficiency curve compared to other mechanisms such as inlet modulation and load/unload operation. This makes VSD compressors useful as trim compressors, supplying the variable demand on top of a stable base demand. However, VSD compressors are not as efficient as FSD compressors at providing a relatively constant output.
A compressed air system that uses both FSD compressors and VSD compressors, however, may result in operational situations where undesirable operation is occurring. Additionally, the FSD compressors may have a limited ability to be repeatedly started and stopped within a given time frame thereby limiting operational flexibility of the compressed air system.